1. Field of the Invention
This invention relates to a method of obtaining a high impact resistant product injection molded from an injection molding material of a styrene-conjugated diene block copolymer with a high styrene content, or a molding material of a resin composition containing a styrene resin other than the styrene-conjugated resin copolymer dispersed in the styrene-conjugated diene block copolymer.
2. Description of the Related Art
In general, styrene-conjugated diene block copolymers with styrene contents of 50% by weight or lower are used as thermoplastic elastomers, and used as modifiers for the general purpose resins and engineering plastics, or base polymers for tackifiers and adhesives ("8883 Chemical Commercial Products", p. 667, Kagaku Kogyo Nipposha (1983). On the other hand, styrene-conjugated diene block copolymers with styrene contents of 50% by weight or more have been used widely as thermoplastic resins (transparent impact resistant polystyrenes) and as alternative for other transparent resins such as PVC resins, PMMA resins, AS resins, MBS resins, and PC resins, or raising the quality of opaque impact resistant resins such as HIPS resins, PP resins, etc. have hitherto been used transparent (Asahi Chemical, Technical Report of ASAFLEX, "Specific Features and Applications of ASAFLEX" p. 78). Among these resins, PVC resins are flame retardant but have a low impact resistance, and a blending of MBS resins is widely practiced to cover the drawback without impairing transparency ("8683 Chemical Commercial Products", p. 527, Kagaku Kogyo Nipposha (1983)). The use of expensive MBS resins, however, increases the cost of the molding material, and a drawback arises in that the disposal of waste materials is difficult so long as PVC resins are employed. This is the reason for using, as a substitute, styrene-conjugated diene block copolymers which are transparent impact resistant polystyrenes, so long as use conditions are allowed. On the other hand, where PMMA resins and AS resins are employed, improvement impact resistance is important, and where MBS resins and PC resins are employed, reduction in cost is important. When obtaining an injection molded product by using such styrene-conjugated diene block copolymers, usually a molding material is injected into an injection molding machine and the molten resin charged into the mold maintained at a temperature lower than the glass transition temperature of the styrene polymer moiety in the block copolymer, to obtain a molded product.
In this injection molding method, however, the impact resistance of the molded product is still too low, and to further expand the use thereof an improved method of obtaining further higher impact resistance while maintaining the transparency of the molded product is required. The present inventors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-91518 that the microphase structure of the styrene-conjugated diene block copolymer must be maintained at a specific orderly structure, to obtain high impact resistance, and a desired stable high impact resistance cannot be obtained even when a specific styrene-conjugated diene block copolymer is injection molded, unless the orderly structure of said block copolymer is made optimum. If the molding of this copolymer is carried out by the compression molding method, with a very low shear rate applied to the molten resin and an orientation of the microphase structure caused thereby, a final molded product having an optimum orderly structure with a very high impact resistance can be obtained. The compression molding method, however, has a low productivity, and a high cost as a method of molding styrene-conjugated diene block copolymers. On the other hand, when the injection molding method generally employed in the industry is used, an highly developed oriented layer is formed near the surface of the molded product due to the influence of the high shear rate during molding, and the microphase structure within the layer is cooled while being oriented, whereby the orderly structure is greatly disturbed, and therefore, it is very difficult to maintain a desirable orderly structure in the final molded product. Therefore, only low Izod impact values are usually obtained in the injection molded products using of the molding material of this copolymer, and a problem arises in that impact resistance is too low for use as substitute for other transparent resins.
It is known to reinforce a styrene-conjugated diene block copolymer with a styrene content of over 50% by weight as the matrix with a styrene type resin (for example, Japanese Examined Patent Publication (Kokoku) No. 60-26429; Koichi Inoue, Plastics, 35(9), 61 (1984), etc.). In this case, dry blending is practiced in the prior art by formulating various styrene resins in styrene-butadiene block copolymers, and after melt blending by a kneading extruder, if desired, the composition is pelletized, the pellets are charged into an injection molding machine, and the molten resin is injected into a mold maintained at a temperature lower than the glass transition temperature of the styrene polymer moiety in the styrene-butadiene block copolymer to obtain a molded product. This is the most widely used modification process.
According to this modification process, however, although the rigidity and heat resistance of the molded product can be improved, but impact resistance is still unsatisfactory in practical application, and to further expand its use, a greatly improved method is required. The present inventors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-91518 that to obtain a high impact resistance, both of the styrene-butadiene block copolymer which forms the matrix and the styrene resin other than said styrene-butadiene block copolymer must keep a specific dispersed state and orderly structure, and even when a specific styrene-butadiene copolymer and a specific styrene resin are used and mixed at a predetermined composition ratio, a desired impact resistance cannot be obtained unless the dispersed states and orderly structures of the styrene-butadiene block copolymer which forms the matrix in the composition and the styrene resin other than the block copolymer are made optimum. Accordingly, the present inventors considered that a final molded product having a high impact resistance could be obtained even by using of the process of the prior art of injection molding after kneading and extrusion of a dry blended product, if the dispersed states and orderly structures of the respective resins are made optimum during kneading and extrusion, and care is taken not to lose the optimum dispersed states and orderly structures during molding. Concerning the kneading and extrusion steps, the present inventors have already filed a patent application relating to a method of preparing pellets, in which the styrene-butadiene block copolymer which forms the matrix and the styrene resin other than the block copolymer keep the most suitable dispersed state and orderly structure (Japanese Unexamined Patent Publication (Kokai) No. 63-278952). When molding such pellets, if molding is performed by the compression molding method with a very low shear rate applied to the molten resin, and an orientation of the matrix and the dispersed phase caused thereby, since the dispersed state and orderly structures in the pellets are substantially preserved, a final molded product having a very high impact resistance can be obtained. The compression molding method, however, has a low productivity and is not practical as the method of molding styrene resin compositions. On the other hand, when the injection molding method generally practiced in industry is used, due to the influence of a high shear rate during molding, an extremely developed oriented layer is formed near the surface of the molded product, and both the matrix and the dispersed phase within the layer are cooled while being oriented, whereby the dispersed state and the orderly structure are greatly disturbed. As a result, it is very difficult to maintain a desirable dispersed state and orderly structure in the final molded product, and a problem arises in that the Izod values of the injection molded product using the pellets are generally low.